J. Taprogge

94β -Decay Half-Lives of Neutron-Rich Cs55 to Ho67 : Experimental Feedback and Evaluation of the r -Process Rare-Earth Peak Formation

The β-decay half-lives of 94 neutron-rich nuclei ^{144-151}Cs, ^{146-154}Ba, ^{148-156}La, ^{150-158}Ce, ^{153-160}Pr, ^{156-162}Nd, ^{159-163}Pm, ^{160-166}Sm, ^{161-168}Eu, ^{165-170}Gd, ^{166-172}Tb, ^{169-173}Dy, ^{172-175}Ho, and two isomeric states ^{174m}Er, ^{172m}Dy were measured at the Radioactive Isotope Beam Factory, providing a new experimental basis to test theoretical models. Strikingly large drops of β-decay half-lives are observed at neutron-number N=97 for _{58}Ce, _{59}Pr, _{60}Nd, and _{62}Sm, and N=105 for _{63}Eu, _{64}Gd, _{65}Tb, and _{66}Dy. Features in the data mirror the interplay between pairing effects and microscopic structure. r-process network calculations performed for a range of mass models and astrophysical conditions show that the 57 half-lives measured for the first time play an important role in shaping the abundance pattern of rare-earth elements in the solar system.

β-decay of neutron-rich Z∼60 nuclei and the origin of rare earth elements

A large fraction of the rare-earth elements observed in the solar system is produced in the astrophysical rapid neutron capture process (r-process). However, current stellar models cannot completely explain the relative abundance of these elements partially because of nuclear physics uncertainties. To address this problem, a β-decay spectroscopy experiment was performed at RI Beam Factory (RIBF) at RIKEN, aimed at studying a wide range of very neutron-rich nuclei with Z∼60 that are progenitors of the rare-earth elements with mass number A∼460. The experiment provides a test of nuclear models as well as experimental inputs for r-process calculations. This contribution presents the experimental setup and some preliminary results of the experiment.

GAMMA-RAY SPECTROSCOPY IN THE VICINITY OF 108 Zr

F. Browne et al.; 4 pags.; 2 figs.; Presented at the Zakopane Conference on Nuclear Physics “Extremes of the Nuclear Landscape”, Zakopane, Poland, August 31–September 7, 2014; PACS numbers: 21.10.Re, 21.10.Tg, 23.20.Js, 27.60.+j

β-decay spectroscopy at RIBF: The EURICA project

EURICA is a project at the RIKEN Nishina Center aimed at studying a wide range of exotic nuclei through β-decay measurements and high-resolution γ-ray spectroscopy. During its first two years of operation, about 80 days of beam time have been dedicated to the project allowing the exploration of several very exotic regions of the nuclear chart, which include key nuclei such as 78Ni, 100Sn, 110Zr, 128Pd, and 138Sn. Data analysis of the EURICA experiments is currently ongoing.

Status and results from the decay spectroscopy project EURICA (Euroball-RIKEN cluster array)

β- and isomer-decay spectroscopy are sensitive probes of nuclear structure, and are often the only techniques capable of providing data for exotic nuclei that are produced with very low rates. Decay properties of exotic nuclei are also essential to model astrophysical events responsible for the evolution of the universe such as the rp- and r-processes. The EURICA project (EUROBALL RIKEN Cluster Array) has been launched in 2012 with the goal of performing spectroscopy of very exotic nuclei. Since 2012, five experimental campaigns have been successfully completed using fragmentation of 124Xe beam and in-flight-fission of 238U beam. In these proceedings we will introduce the experimental setup and highlight some key recent results around 78Ni, 132Sn, and 110Zn published during 2014 and 2015.

Is seniority a partial dynamic symmetry in the first ν g 9/2 shell?

Abstract The low-lying structures of the midshell ν g 9 / 2 Ni isotopes 72Ni and 74Ni have been investigated at the RIBF facility in RIKEN within the EURICA collaboration. Previously unobserved low-lying states were accessed for the first time following β decay of the mother nuclei 72Co and 74Co. As a result, we provide a complete picture in terms of the seniority scheme up to the first ( 8 + ) levels for both nuclei. The experimental results are compared to shell-model calculations in order to define to what extent the seniority quantum number is preserved in the first neutron g 9 / 2 shell. We find that the disappearance of the seniority isomerism in the ( 8 1 + ) states can be explained by a lowering of the seniority-four ( 6 + ) levels as predicted years ago. For 74Ni, the internal de-excitation pattern of the newly observed ( 6 2 + ) state supports a restoration of the normal seniority ordering up to spin J = 4 . This property, unexplained by the shell-model calculations, is in agreement with a dominance of the single-particle spherical regime near 78Ni.

Systematic Study of β-Decay Half-Lives in the Vicinity of 78Ni

Experimental study of β-decay properties for nuclei around Ni is of great importance as Ni is probably the most neutron-rich nuclide known so far. Observables like β-decay half-life (T1/2), β-decay Q value (Qβ), neutron emission probability (Pn) can serve as benchmarks for one to understand the nuclear structure far from the valley of stability. Results from the experiment allow for systematic study and comparison between different global mass models or various theories including shell model calculations, which will largely extend the present knowledge of nuclear structure towards neutron drip-line. In order to produce the secondary beam containing very neutron-rich nuclei around Ni, the experiment was performed as part of EURICA campaign at Radioactive Isotope beam factory (RIBF) operated by RIKEN Nishina Center (RNC) and Center of Nuclear Study (CNS), University of Tokyo. A high intensity U beam was extracted from accelerator complex at RIBF with an energy of 345 MeV/nucleon. The secondary beam was generated by the in-flight fission of U in a 3 mm Be target. The fragments from fission of U were analyzed in the BigRIPS and ZeroDegree with an event-by-event mode. A highly segmented active stopper, WAS3ABi, was mounted after the last focal plane of ZeroDegree spectrometer, F11, for the implantation of fragments of interest transimitted through BigRIPS and ZeroDegree. The ion-β-γ correlation was obtained to study the half-lives as well as β-delayed γ-ray spectroscopy of exotic nuclei around Ni. In this presentation, new half-lives of Co, Co, Ni, Cu, Cu, Zn, Ga, and Ge, which was measured for the first time experimentally, will be reported. Half-life of nuclide such as Ni, which had been measured in previous experiment but resulted in a large uncertainty, was re-measured in this work with a much higher precision. The shell gap effect on half-lives and magicity at Z = 28 and N = 50 will be discussed based on the data from this experiment together with previous ones from literature. APPC12 The 12th Asia Pacific Physics Conference

Isomeric states in neutron-rich In-129 and the pi g(9/2)(-1) circle times nu h(11/2)(-1) multiplet

Within the RISING stopped beam campaign the neutron-rich indium isotopes with masses A=125-130 have been studied using the method of isomer spectroscopy. The decays of several isomeric states have been observed and here we compare our results for 129In to previous measurements for this nucleus. The isomeric states were populated in the fragmentation of a 136Xe beam at 750 MeV/u and in the relativistic fission of a 238U beam at 650 MeV/u at the accelerator facility GSI (Darmstadt, Germany).4 pags.; 5 figs.; 1 tab.; XX International School on Nuclear Physics, Neutron Physics and Applications (Varna2013): Open Access funded by Creative Commons Atribution Licence 3.0

Isomeric states in neutron-rich 129In and the πg−19/2 ⊗ vh−111/2 multiplet

Within the RISING stopped beam campaign the neutron-rich indium isotopes with masses A=125-130 have been studied using the method of isomer spectroscopy. The decays of several isomeric states have been observed and here we compare our results for 129In to previous measurements for this nucleus. The isomeric states were populated in the fragmentation of a 136Xe beam at 750 MeV/u and in the relativistic fission of a 238U beam at 650 MeV/u at the accelerator facility GSI (Darmstadt, Germany).4 pags.; 5 figs.; 1 tab.; XX International School on Nuclear Physics, Neutron Physics and Applications (Varna2013): Open Access funded by Creative Commons Atribution Licence 3.0